A liquid crystal display (LCD) is a passive display that is not able to emit light by itself. Accordingly, it requires to use a source of light to project light through the display panel in order to display all sorts of information.
The source of light may be supplied by a backlight unit. The backlight unit functions as a component of liquid crystal display device to guide light to the display panel in apparatus such as liquid crystal display monitors, portable computers, and navigation systems for vehicles.
With the needs of a large sized display area and a small sized system, a backlight unit has been changed from rear-disposed lamp type in which a lamp is disposed at the rear to side-disposed lamp type in which a lamp is disposed at the side. A backlight unit of side-disposed lamp type generally includes a mold frame, a light source such as a cold cathode fluorescent tube of 23 mm in diameter, a reflecting sheet for reflecting ahead light projected from the light source, a light guide panel for guiding ahead light reflected from the reflecting sheet, a diffusing sheet for diffusing uniformly light guided through the light guide panel, a prism sheet for enhancing the luminance of light passed through the diffusing sheet, and a protecting sheet for protecting the prism sheet. The reflecting sheet, light guide panel, diffusing sheet, prism sheet and protecting sheet are mounted in grooves of the mold frame. The light guide panel is a plate-shaped body made of transparent materials such as acrylic resin, and has the shape of which thickness at the portion thereof remote from the light source is getting gradually smaller than that of the portion thereof adjacent to the light source.
In order to fix the reflecting sheet, light guide panel, diffusing sheet, prism sheet and protecting sheet to the grooves of the mold frame, method using an adhesive or method forming light guide projections and the other grooves respectively in the light guide panel and in left and right sides of the mold frame to fit the light guide projections to the grooves has been used. However, in the latter case the light guide projections have right-angled corners, reflection and transmission of light are repeated in space between the light guide panel and the mold frame so that light may be concentrated at the right-angled corners of the light guide projections of the light guide panel. As a result of this concentration, the light guide projections of the guide panel may be brighter compared to other portions. The concentrated light may result in a poor quality LCD.
To solve this problem, there has been proposed a conventional backlight unit as disclosed in U.S. Pat. No. 5,988,827. In the backlight unit, corners of light guide projections remote from a lamp are rounded. Grooves of a mold frame corresponding to the shape of the rounded corners of the light guide projections are also rounded. By virtue of this structure, the rounded corners of the light guide projections can efficiently prevent light from being concentrated. However, it is still possible that right-angled recess portions between the light guide projections and a body of a light guide panel can be cracked when an LCD module is shaken or impacted. FIG. 1 is a perspective view illustrating a conventional light guide panel cracked by impact in direction Y. When the light guide projections is impacted by shaking of the LCD module, stress is concentrated on point P in the light guide panel. The reason is that stress at point P is about 27% larger than that at point Q. If stress at point P exceeds the yield stress, a crack 14 can be developed from point P. The crack 14 may have the light guide panel and the display panel render a poor image, resulting in a poor quality LCD. Also, if the light guide panel is excessively cracked, the light guide projections can be disengaged from the grooves such that the light guide panel can be slid to the lamp and damage it.
Accordingly, it needs an improved backlight unit having an anti-impact light guide panel for the LCD module for a mobile apparatus such as portable computers and personal digital assistants (PDAs) that are frequently exposed to impacts by falling or external forces compared to other display devices.